LP2008918: default modal background color

LP2008918: default modal background color

To test:
1) Open the holdings editor in the angular staff catalog
2) right click on an item
3) select Add/Manage Item notes
4) Note that you can't see the text at the top of the modal, nor can you see the close button
5) Apply this patch and repeat steps 1-3
6) Note that the modal header is a darker color, so the text and close button are again visible.

Signed-off-by: Jane Sandberg <js7389@princeton.edu>